1. Field of the Invention
The present invention relates to an anti-lock controller, and more particularly to an anti-lock controller for improving the safety of a vehicle by preventing a wheel from becoming locked when brakes are applied to the vehicle.
2. Description of the Related Art
if a brake pedal or the like is operated during the traveling of the vehicle, the braking torque of a magnitude corresponding to the amount of the pedal or the like operated is applied to the wheels by a braking device such as a hydraulic braking device. The wheel speed (the rotating speed of the wheel) becomes lower than the vehicle speed due to this braking torque (undergoes so-called slip), so that sliding friction occurs between the wheel and the road surface. This sliding friction serves as a braking force, and the vehicle is thereby decelerated or brought to a standstill. The slip rate which represents the degree of slip is defined by the following Formula (1) if it is assumed that a vehicle speed is V.sub.GS, and that a wheel speed is V.sub.W : ##EQU1##
The braking force is dependent upon a coefficient of braking friction .mu. (hereafter simply referred to as ".mu.") acting between the wheel and the road surface, and .mu. changes in correspondence with the slip rate S. On dry asphalt, for example, .mu. changes substantially in proportion to the slip rate S when the slip rate is low, as shown in FIG. 1A, and .mu. reaches a peak (the braking force also reaches a peak) when the slip rate S is at a predetermined value (generally, 8 to 30% or thereabouts; S.sub.R shown in FIG. 1A). If a still higher braking torque is applied and the slip rate S exceeds the predetermined value S.sub.R, .mu. declines (and the braking force also declines correspondingly). With a decline in .mu., the braking force declines, which results in an increase in the braking distance, a substantial decline in the cornering force of the tire due to the slip rate, and the like. Hence, the stability of the vehicle declines. Then, if a higher braking torque is applied, the wheel becomes locked (slip rate S=1), thereby resulting in the loss of the directional stability and steerability of the vehicle.
To prevent the wheel from becoming locked during the braking of the vehicle and bring the vehicle to a standstill in a short braking distance, anti-lock controllers, i.e., anti-lock braking systems, of various arrangements have hitherto been proposed. For example, there has been proposed an anti-lock controller in which a slip rate at which .mu. reaches a peak is fixedly set in advance as a targeted slip rate, a targeted wheel speed is computed on the basis of a vehicle speed estimated from the wheel speed and the targeted slip rate, and the braking torque is controlled such that the wheel speed becomes the targeted wheel speed. However, since the slip rate at which .mu. reaches a peak varies depending on the condition of the road surface and the like, even if the braking torque is controlled on the basis of the slip rate which is fixedly set as described above, it does not necessarily follow that an optimal braking state is always obtained under various conditions of the road surface.
In addition, Japanese Patent Application Laid-Open No. 56-53943 discloses a technique wherein the peak of .mu. is detected on the basis of a change in the wheel speed while controlling the braking torque in predetermined periods during the braking of the vehicle, a tendency of the change in the targeted wheel speed is determined on the basis of respective wheel speeds V.sub.W1 and V.sub.W2 at a point of time when two peaks of .mu. have been detected, and the targeted wheel speed during an ensuing control period is determined by extrapolation on the basis of that tendency.
However, the wheel speeds V.sub.W1 and V.sub.W2 at the peaks of .mu. include detection errors, and these errors increase as the time interval of the detection of the wheel speeds V.sub.W1 and V.sub.W2 becomes large. Hence, to reduce the effect of the errors, it is necessary to shorten the control period, and increase the frequency of detection of the peaks of .mu.. In addition, in the above-described publication, it is stated that the wheel is subjected to skid control in a short control period, the acceleration of the wheel, the wheel load, and the braking torque existing at that time are detected, and an equation of motion is solved, so as to detect a peak of .mu.. However, with the above-described method of detecting the peak of .mu., as is apparent from FIG. 5 of the publication, the peak of .mu. cannot be detected unless the magnitude of the braking torque is changed with respect to the peak of .mu. such that .mu. changes to some extent. Accordingly, there has been a problem in that the braking performance deteriorates in detecting the peak of .mu..